Such a sample sensor is known from U.S. Pat. No. 6,552,792. This US patent describes a wavelength modulated photo acoustic spectrometry system and method for measuring a concentration of a trace gas in a gas mixture. The method comprises generating light from a light source; passing the light through a sample area; sampling sound produced by the light passing through the sample area with an acoustic detector; and controlling the wavelength of the light with a wavelength controller. The wavelength modulation is performed with a frequency f, around the optimal absorption wavelength of the trace gas. When, due to temperature variation, the average wavelength of the light from the light source changes, the wavelength shift is detected. A wavelength controller compensates for this effect by adjusting the average wavelength.
The temperature compensation is performed as follows. Part of the light passes through a reference gas cell with a relatively high concentration of the trace gas. A photo diode behind the reference cell provides a signal that depends on the absorption in the reference cell. The wavelength modulation with a frequency f results in variations in the photo diode signal with a frequency 2f (two absorption peaks for every modulation period). When, due to a temperature change, the wavelength of the light produced by the light source is shifted, the modulation is not performed exactly around the optimal absorption value anymore. As a result, odd harmonics (3f) of the modulation frequency are introduced in the detector signal. The 3f signal on the photo detector varies when the average wavelength shifts through the absorption spectrum of the trace gas and equals zero when the average laser wavelength corresponds to the centre of the absorption line. The wavelength controller adjusts the average wavelength of the light source in order to minimize the 3f component in the photo diode signal.
Although this approach can be used for fundamental studies in research institutes, the use of a reference gas cell is not a preferred approach for a commercial product. Because the reference gas cell signal is based on absorption, a long reference gas cell is required which is not attractive for a compact sensor or a high concentration needs to be used which in case of, for example, NO2 becomes dangerous when the reference gas cell accidentally breaks. Furthermore it is a problem of the reference gas cell, that the concentration of some gases is not stable over long periods due to for instance wall sticking or dissociation.